1. Field of the Invention
The present invention relates to a fuel control system for vehicle engines, and, more particularly, to a fuel control system including fuel vapor control system which estimates, or otherwise measures, an amount of fuel vapors stored in a storage canister and calculates an amount of fuel vapors purged into an intake system based on the estimated or measured amount of fuel vapors.
2. Description of Related Art
Typically, fuel injection systems for automobiles cooperate with fuel control systems which determines a proper air-to-fuel ratio of an air-fuel mixture based on an amount of intake air introduced into an intake system. Based on the amount of intake air, an injector is pulsed at a basic pulse width. However, there is a limit in the accuracy of fuel mixture setting control. Fuel from an injector is not always immediately delivered entirely into an engine. Further, the injector valve suffers changes in injection characteristics due to aging. For these reasons, if the amount of fuel delivered by a given injector is determined based on the amount of intake air only, it is hard to deliver an air-to-fuel ration agreeable with the target air-to-fuel ration with a high accuracy. For more accurate air-to-fuel control, a closed-loop or feedback control system has an oxygen sensor for monitoring the content of oxygen in the exhaust to verify the accuracy of the mixture setting. the oxygen content is off, the system corrects itself to bring the oxygen back to proper levels. The system tries to maintain a target air-to-fuel ratio which refers to an ideally combustible air-fuel mixture. Whenever the oxygen content is off, the system corrects itself to bring the oxygen back to proper levels. The system tries to maintain a target air-to-fuel ratio which refers to an ideally combustible air-fuel mixture. If the feedback control parameter remains a fixed level, an air-to-fuel ratio is controlled in an open-loop.
Automobiles are also provided with evaporation control systems. Such an evaporation control system as emission control systems designed to prevent gasoline vapors escaping into atmosphere from a fuel tank. A vapor storage canister is filled with highly activated charcoal particles or granules for absorbing and storing fuel vapors when the fuel vapors touch them. The evaporation control system includes a purge device for delivering properly fuel vapors into the intake system. In such an evaporation. system, the vapor storage canister is connected to the intake system through a purge line with a purge valve. When the purge valve opens, fuel vapors are introduced into the intake system from the vapor storage canister. If, while an air-to-fuel ratio is controlled in an open-loop, vapor purging takes place, the air-to-fuel ratio shifts greatly from the target air-to-fuel ratio. Accordingly, vapor purging is ordinarily effected during feedback control. In such a case where vapor purging is effected during feedback control, purged vapors are regarded as a disturbance in air-to-fuel ratio control. If the vapor storage canister stores fuel vapors, this disturbance is compensated by changing a feedback control parameter to a lean side from a neutral level in the feedback air-to-fuel ratio control. While, if the amount of fuel vapors delivered into the intake system is constant and the engine operates under ordinary driving conditions, the compensation of a disturbance such as due to purging is exact, nevertheless, if there occurs a sudden change in the among of purged fuel vapors, for example if the purge valve is opened from a shut down state or closed from an opened state, or otherwise there is a sudden pressure drop between before and after the purge valve, or the engine is in transient states of operation such as acceleration and deceleration, the compensation of disturbances such as due to purging is insufficient due to a delay of detection of an air-to-fuel ratio or a delay of response in the feedback air-to-fuel ratio control, leading to a great shift of air-to-fuel ratio from the target air-to-fuel ratio. Such a shift of air-to-fuel ratio is considered to result from some reasons.
If the purge valve is opened from a shut down state during the feedback air-to-fuel ratio control, an air-to-fuel ratio is changed so as to enrich a fuel mixture. This air-to-fuel ratio is monitored by a linear oxygen (O.sub.2) sensor in the exhaust line and controlled to change toward a lean side so as to become a proper level. when the purging is one of causes of an enriched air-to-fuel ratio, there doe not arise any correction of the enriched air-to-fuel ratio until a change in air-to-fuel ratio is actually monitored by the oxygen sensor or a correction of the enriched air-to-fuel ratio takes place with a delay of time. Further, when the engine is in a transient state of operation such as acceleration during purging, there occurs a sudden change in the pressure difference between before and after the purging valve. As a result, the amount of fuel vapors itself or a proportion of the amount of fuel vapors relative to the total amount of fuel, introduced into the engine for one intake stroke, drops suddenly, resulting in a lean air-to-fuel ratio. On the other hand, on deceleration during purging, an air-to-fuel ratio is enriched. Neither the lean air-to-fuel ratio nor the rich air-to-fuel ratio is corrected until it is monitored by the oxygen sensor. Accordingly, fuel is consumed more than necessary and hydrocarbon emission into atmosphere increases in the incident where the rich air-to-fuel ratio remains for a while or is corrected with a delay of time. On the other hand, in the incident where the air-to-fuel ratio remains lean, the engine can not provide sufficient output.
If an increasing change and a decreasing change in the amount of purged fuel vapors alternately takes place at frequent intervals, or if acceleration and deceleration are repeated at frequent intervals during purging, the air-to-fuel ratio feedback control takes effect with a delay of time and consequently, causes hunting, so as to turn out unstable. In the case where purged fuel vapors are treated as disturbances against the air-to-fuel ratio feedback control, if a large among of fuel vapors is purged, the feedback control parameter clings to a limit on the lean side as a result that the air-to-fuel ratio control system tries to counteract the disturbances, leading to failure in meeting disturbances caused for other reasons.
It can be thought to detect the amount of purged fuel vapors and use the purged fuel vapors as a part of a substantially necessary amount of fuel, so as thereby to exclude the purged fuel vapors acting as disturbances against the air-to-fuel ratio feedback control. However, there has not been any practical approaches to detect directly the amount of purged fuel vapor. Accordingly, approaches have been made of indirect detection of the amount of purged vapors.
One such approach is that described in Japanese Laid-Open Patent No. 2-245441. The approach used was to estimate the purged amount of fuel vapors based on a difference of a feedback control parameter from a neutral level. In this prior art fuel system, a purged amount of fuel vapors per one revolution of engine is calculated as the estimated pursed amount of fuel vapors, the basic amount of fuel delivered per one revolution of engine by a given injector is reduced by the purged amount of fuel vapors.
As has been proved in the art, the purged amount of fuel vapors changes at short intervals with changes in engine driving conditions including, for instance, the amount of intake air, the pressure of intake air and the speed of engine. Together, because, as was previously described, the calculation of a feedback control parameter is based on an air-to-fuel ratio monitored by an oxygen sensor and consequently, accompanied by a delay of time, if the engine driving condition, i.e. the actual purged amount of fuel vapors, changes at short intervals, the estimation of the purged amount of fuel vapor is made with only a low accuracy, leading to a great shift of air-to-fuel ratio from the target air-to-fuel ratio.
From the above discussions, shifts in air-to-fuel ratio from a target air-to-fuel ratio can be avoided during purging if the purged amount of fuel vapors is detected with a high accuracy. As a result of much attention having been given to various approaches relating to high accuracy detection of the purged amount of fuel vapors, it has been proved that changes in the amount of fuel vapors stored in a vapor storage canister due to the passage of time are notably lenient as compared to changes in engine driving conditions and are insignificant in a period of time equivalent to the delay of response of the air-to-fuel feedback control to monitored air-to-fuel ratios or in one cycle of the air-to-fuel feedback control. This teaching alludes to a technique for detecting the purged amount of fuel vapors with a high accuracy without accompanying a delay of time due to the detection of air-to-fuel ratio.